The present invention relates to a chip positive temperature coefficient (hereinafter, PTC) thermistor comprising conductive polymers having PTC properties. The present invention particularly relates to a laminated chip PTC thermistor and a method of manufacturing the same.
PTC thermistors have been used as an overcurrent protection element. When an electric circuit gets overloaded, conductive polymers of a PTC thermistor, which have PTC properties, emit heat and thermally expand to become high resistance, thereby reducing the current in the circuit to a safe small current level.
The following is a description of a conventional laminated chip PTC thermistor (hereinafter, PTC thermistor).
Conventional PTC thermistors include those disclosed in the Japanese Patent Application Laid Open Publication No. S61-10203 in which a thermistor is constructed such that a plurality of conductive polymer sheets and metal foils are alternately laminated and terminals are provided on opposite side faces.
FIG. 11(a) is a cross section of a conventional PTC thermistor. FIG. 11(a) shows conductive polymer sheets (hereinafter, polymer sheet) 1a, 1b and 1c. Electrodes 2a, 2b, 2c and 2d sandwich the polymer sheets 1a, 1b and 1c such that openings 3 are formed on alternate sides of the electrodes 2a, 2b, 2c and 2d. By layering these electrodes 2a, 2b, 2c and 2d, and the polymer sheets 1a, 1b and 1c alternately, a laminate 4 is formed. On side faces of the laminate 4 are terminals 5a and 5b. 
However, the construction of the conventional PTC thermistor has problems: due to the considerably large differences in thermal expansion coefficients between the component materials, mechanical stress applied during operation of the PTC thermistor has caused cracks in and degraded the connection points between the electrodes 2a, 2b and 2c and the terminals 5a and 5b. In some severe cases, such degradation resulted in breaking of wires
Furthermore, since terminals 5a and 5b fail to extend either to the bottom face of the polymer sheet 1c or to the top face of the polymer sheet 1a, the PTC thermistor can not be mounted on a flat surface. To address this problem, the terminals 5a and 5b need to be extended to the lowest point of the polymer sheet 1c and the highest point of the polymer sheet 1a respectively. FIG. 11(b) shows a cross section of a PTC thermistor which has been modified as mentioned above and soldered on a printed circuit board. In this construction, when the PTC thermistor is soldered on the printed circuit board, large differences in the thermal expansion coefficients among the polymer sheets 1a, 1b and 1c, the electrodes 2a, 2b, 2c, and 3d, and the terminals 5a and 5b cause the terminal 5a in particular to be distorted. Due to this distortion, when the soldering is carried out stress remains on the bonded surfaces between the terminal 5a and the polymer sheet ic and the contact section between the terminal 5a and the electrode 2c. The PTC thermistor serves as a protection device against overload: its conductive polymers expand under heat and become a high resistance. While the PTC thermistor is in a protection operation, the thermal expansion of the polymer sheet 1a, 1b and 1c causes a significant mechanical stress. Repeated protection operations, namely, repeated expansion and shrinkage of the conductive polymers promote separation of the bonded surfaces between the terminal 5a and the polymer sheet 1c. Further, since stress concentrates on the connection point between the terminal and the electrode, the connection point suffers cracks, which in some cases, triggers breaking of wires.
The present invention aims at solving the foregoing problems of the conventional laminated PTC thermistors and providing a chip PTC thermistor which does not suffer cracks in the connection sections between the inner electrodes and side electrodes, achieve a long-term reliable connection and is suitable for surface mounting.
The chip PTC thermistor of the present invention comprises:
a) a conductive polymer having PTC properties;
b) a first outer electrode in contact with the conductive polymer;
c) a second outer electrode sandwiching the conductive polymer with the first outer layer electrode;
d) one or more inner electrode disposed in between and parallel to the first and second outer electrodes and sandwiched between the conductive polymer;
e) a first electrode electrically directly coupled with the first outer electrode; and
f) a second electrode disposed electrically independently from the first electrode.
Where, when counting from one inner electrode, which is the closest to the first outer electrode, an inner electrode in xe2x80x9cnxe2x80x9dth position is called as the xe2x80x9cnxe2x80x9dth inner electrode. And when xe2x80x9cnxe2x80x9d is an odd-number, the inner electrodes are directly coupled with the second electrode, and when xe2x80x9cnxe2x80x9d is an even-number, the inner electrodes, with the first electrode. If the total number of the inner electrodes is an odd number, the second outer electrode is electrically directly coupled with the first electrode, and if the total number of the inner electrodes is an even number, with the second electrode. In this PTC thermistor, the cross sections where the odd-numbered inner electrodes are coupled with the second electrode are thicker than the other sections, and the cross sections where the even numbered inner electrodes are coupled with the first electrode are thicker than the other sections.
According to this construction, even if repeated expansion and shrinkage of the conductive polymers impose stress, connection points between inner electrodes and side face electrodes do not suffer cracks. Thus, the PTC thermistor of the present invention achieves a superior long-term connection reliability and is suitable for surface mounting.